feat: redesign Todos tab — grouping, collapsible headers, cleaned columns

Implements the approved mockup from the UX debate report:

- new DisplayRow::TodoHeader variant: collapsible group headers, toggled
  by Enter/Space, tracked in HubState.todo_group_collapsed
- [g] cycles grouping: status (default) -> project -> repo -> none
- [s] cycles which statuses are included before grouping: active
  (open+in-progress, default) -> open -> done -> all
- table: dropped SRC/EST (near-always empty), code-scan todos show a dim
  'no status icon' marker instead of a separate source column, project
  name colored by the project's own color (was DarkGray, wasting the most
  useful visual identifier in the tool), age now tracks updated_at not
  created_at ('what's moving' beats 'what's old')
- right panel: added Repo/Difficulty/Estimate, previously only visible in
  the table despite the panel having more room for them
- block title shows live counts + current view/group/code-scan state

Simplified vs. the literal ASCII mockup: [s] filters the underlying set
before grouping, so e.g. the default 'active' view omits Done/Cancelled
groups entirely rather than always showing them collapsed - a simpler,
more internally consistent model. Flagged for foksiu to request the exact
mockup behavior if he wants it.

Verified via pyte: group/status-view cycling, collapse/expand (arrow flips
and child rows appear/disappear), right panel on header vs. todo row.

src/hub/modals_spawn.rs

@@ -618,10 +618,13 @@ pub fn open_action_menu(state: &mut HubState) {
     match state.tab {
         Tab::Todos => {
             entries.push(entry("Cycle todo status",   "Space", kc(KeyCode::Char(' '))));
+            entries.push(entry("Expand/collapse group","↵",    kc(KeyCode::Enter)));
             entries.push(entry("Edit todo",           "e",     k('e')));
             entries.push(entry("Delete todo",         "Del",   kc(KeyCode::Delete)));
             entries.push(entry("New todo",            "+",     k('+')));
-            entries.push(entry("Toggle code todos",   "X",     k('X')));
+            entries.push(entry("Cycle status view",   "s",     k('s')));
+            entries.push(entry("Cycle grouping",      "g",     k('g')));
+            entries.push(entry("Toggle code todos",   "c",     k('c')));
             entries.push(entry("Scan repos for TODOs","T",     k('T')));
         }
         Tab::PRs => {